Surgical needles can be used in a variety of percutaneous medical procedures, such as diagnosing a condition (e.g., biopsy) and delivering a treatment or therapy (e.g., drug delivery, thermal ablation (heat or cold), brachytherapy, and targeted doses of chemotherapy or radiotherapy) to a target location in the patient. The effectiveness of such diagnoses and treatments is strongly correlated with the accuracy with which the needle tip is placed at the desired target location. Conventional free-hand needle placement involves manipulating a visualization system (e.g., an ultrasound probe) while simultaneously inserting and advancing the needle, which requires the physician to mentally relate the images on a video screen to locations inside the patient.
There are numerous factors that can contribute to needle placement inaccuracy. Example factors include tissue deformation, registration error, and the surgeon's hand-eye coordination. There are also situations where a straight-line path to the surgical site is not possible because of anatomical constraints. Examples include deep brain stimulation, where certain targets can be obstructed by eloquent brain tissue and transperineal prostate brachytherapy, where the pubic arch can sometimes obstruct a portion of the prostate. It may also be desirable to reach multiple sites within the target area without full retraction of the needle. In these situations, it may be desirable to steer the needle to navigate a desired needle trajectory.
Surgical needles have a beveled tip that generates bending forces as the needle is advanced into tissue. If the needle is flexible, the bending forces generated by the beveled tip cause the needle to follow a curved path. These bending forces can be harnessed to steer the needle to a target location in the tissue. Steering is achieved by controlling the insertion/advancing velocity of the needle (i.e., along the longitudinal axis of the needle) and the rotation velocity of the needle (i.e., about the longitudinal axis of the needle). Control of these variables is achieved by duty cycling.
Duty cycling adjusts the curved trajectory of the advancing needle by alternating between periods of insertion without rotation and periods of insertion with rotation. When the flexible bevel tip needle is inserted without rotation, the needle follows a trajectory with some natural curvature that is dependent on needle characteristics (e.g., stiffness and bevel angle) and tissue characteristics (e.g., density, consistency, homogeneity). When the needle is advanced with a sufficient rotational velocity, straight trajectories can be achieved. Trajectories ranging from naturally curved to straight (zero curve) can be achieved by combining periods of rotation with periods of non-rotation, i.e., duty cycling.
In the field of steerable surgical needles, “duty cycling” refers to the amount of time that the needle rotates stated as a percentage of total needle advancement time. Thus, for example, if the needle is advanced with a 25% duty cycle, it would be advanced under rotation 25% of the time and without rotation 75% of the time. Duty cycling the needle can be periodic in that the total advancement time can be broken down in to periods, e.g., seconds or fractions thereof, and the duty cycling can occur within those periods.
Robotic systems are ideally suited for duty cycling steerable needles because of their ability to apply constant insertion/advancing and rotational velocities. Robotic systems also have the potential to improve the accuracy of needle tip placement through accurate needle alignment using spatial registration, and can perform tasks rapidly with both accuracy and repeatability. However, even perfect pre-entry alignment cannot guarantee accurate tip placement. Errors can arise due to unforeseen variables, such as deflection at membranes, tissue deformation, non-homogeneous tissue, registration and calibration tolerances, etc. The only way to compensate for these errors is to correct the trajectory by steering the needle. Additionally, the needle can be steered to maneuver around sensitive structures to reach locations where straight trajectories may not be feasible or desirable.
Steering the surgical needle by duty cycling lends well to robotic systems because these systems can implement control algorithms that adjust the duty cycle to steer the needle along desired trajectories. This robotic needle steering can be performed, for example, using image guidance feedback to monitor performance and to calculate adjustments in real time.
Straight beveled tip flexible shaft needles may not yield the degree of curvature necessary to effectuate a desired degree of steerability. To improve the curvature, kinked tip needles have been configured with a bend or kink near the beveled tip. The natural trajectory of a kinked beveled tip needle has a higher degree of curvature than the straight beveled tip needle. This higher degree of curvature improves the steerability of the needle through duty cycling.
Kinked tip needle configurations do have some drawbacks. Since straight trajectories are achieved through simultaneous needle rotation and longitudinal advancement, and due to the rigid bend in the needle shaft, the kinked tip cuts a helical path through the tissue. This helical path has a diameter that is larger than the diameter of the needle shaft and can thus cause undesirable damage to the tissue.